Developing method for electrophotography

ABSTRACT

A DEVELOPING METHOD FOR ELECTROPHOTOGRAPHY COMPRISING THE STEPS OF DEVELOPING AN ELECTROSTATIC LATENT IMAGE ON A PHOTOCONDUCTIVE INSULATING LAYER WITH A LIQUID DEVELOPING AGENT CONTAINING THEREIN FINE PARTICLES HAVING A POSITIVE ELECTRICAL CHARGE AND SUBSEQUENTLY CLEANING SAID INSULATIVE LAYER CARRYING THE IMAGE THEREON, THE IMPROVEMENT WHICH COMPRISES SAID LIQUID DEVELOPING AGENT BEING COMPRISED OF A CARRIER LIQUID, FINE PARTICLES HAVING A POSITIVE ELECTRIC CHARGE CONTAINED IN SAID CARRIER LIQUID IN THE DISPERSED STATE, AND A RESIN CONTAINDE IN SAID CARRIER LIQUID IN THE DISSOLVED STATE (I) SAID CARRIER LIQUID HAVING AN ELECTRIC RESISTANCE EXCEEDING 1011 $ CM. AND BEING INSOLUBLE IN ETHYL CELLULOSE, (II) SAID FINE PARTICLES WHICH ARE CONTAINED IN THE DISPERSED STATE BEING COMPOSED OF (A) PIGMENT PARTICLES INSOLUBLE IN THE CARRIER LIQUID AND (B) ETHYL CELLULOSE PARTICLES, AND (III) AT LEAST A PART OF THE RESIN WHICH CONTAINED IN THE DISSOLVED STATE IN THE SAID LIQUID BEING INSOLUBLE IN A CLEANING LIQUID USED IN SAID CLEANING.

United States Patent 3,703,400 DEVELOPING METHOD FOR ELECTRO- PHOTOGRAPHY Yasuo Tamai, Masamichi Sato, Seiji Matsumoto, and Satoru Honjo, Saitama, Japan, assignors to Fuji Photo Film Co., Ltd., Kanagawa, Japan No Drawing. Filed Apr. 7, 1970, Ser. No. 26,442 Claims priority, application Japan, Apr. 7, 1969, 44/26,263 Int. Cl. B41c 1/06; G03g 9/04 US. Cl. 117-37 LE 1 Claim ABSTRACT OF THE DISCLOSURE A developing method for electrophotography comprising the steps of developing an electrostatic latent image on a photoconductive insulating layer with a liquid developing agent containing therein fine particles having a positive electrical charge and subsequently cleaning said insulative layer carrying the image thereon, the improvement which comprises said liquid developing agent being comprised of a carrier liquid, fine particles having a positive electric charge contained in said carrier liquid in the dispersed state, and a resin contained in said carrier liquid in the dissolved state (I) said carrier liquid having an electric resistance exceeding 10 cm. and being insoluble in ethyl cellulose, (II) said fine particles which are contained in the dispersed state being composed of (a) pigment particles insoluble in the carrier liquid and (b) ethyl cellulose particles, and

(III) at least a part of the resin which is contained in the dissolved state in the said liquid being insoluble in a cleaning liquid used in said cleaning.

BACKGROUND 'OF THE INVENTION (1) Field of the invention Generally in electrophotographic processes, numerous developing methods are employed. Of the many methods, the liquid developing method provides images of high quality and therefore is suitable for the reproduction of images of continuous tone. The liquid developing method yields excellent results, in cases where a multicolor image is obtained, by repeating the development using developing agents of diiferent colors on one sensitive layer, such as an electrophotographic sensitive layer prepared with photoconductive zinc oxide (hereinafter this process is referred to as the combination developing method). Generally, a liquid developing agent is com- 3,703,400- Patented Nov. 21, 1972 posed of a highly insulating carrier liquid having very fine particles with suitable sign and quantity of electric charge suspended therein. In order for the agent to exhibit excellent electrophotographic properties, it is necessary that the electric charge of particles be so stable that the electric charge of the suspended particles in the carrier liquid will not vary in magnitude or will not be inverted in sign after a prolonged period of storage.

Further, with the combination developing method, it is necessary that the suspended particles should have a like sign and substantially the same magnitude of electric charge in different developing agents. In actuality, however, it has been fairly difiicult to prepare such developing agents so as to satisfy these requirements.

One drawback of the liquid developing process in that excess developing agent remains deposited on the sheet surface after development, and, if allowed to dry in that state, it will result in fogging. An efiective method of eliminating this fogging is disclosed in the specification of Japanese patent publication No. SHO 36/ 19060. Accordingly, the practice of cleaning the electrophotographic sensitive layer, subsequent to the step of development with a liquid (not containing a toner) similar to the carrier liquid used in the liquid developing agent is already known.

This method suffers from the following shortcomings.

Where the developing, cleaning and drying steps are mechanized through incorporation of necessary devices, the image is observed to be destroyed or blurred (streaked) when a cleaning liquid contacts the developed surface of the sheet at a comparatively large flow rate so as to reduce the length of the developing time in the course of cleaning or when the sheet is passed through squeeze rollers so as to remove the cleaning liquid adhering thereto immediately after the cleaning step. This disadvantage is hardly observed during treatment performed at a lower rate (namely, a treatment consuming an ample amount of time).

When a container tray is filled with a cleaning liquid and the developed sheet is immersed in the bath with the developed surface facing downward and then shaken gently in the hath, no destruction of the image is experienced even with the cleaning bath described by the aforementioned patent. Additionally it has been found that if the image density should decrease, it will take place evenly and therefore will not prove disadvantageous in many cases.

Where an image of continuous tone with sufficiently high density is desired to be obtained through a high-speed treatment, treatment of the sheet through squeeze rollers following the step of cleaning is found to be indispensable. For this reason, the disadvantage in high-speed treatment must be eliminated together with the aforementioned drawbacks of liquid developing agent.

Thus, an object of the present invention is to provide a method which proves particularly advantageous for the electrophotographic developing method used at a high rate of speed. In such use, as for producing multi-color prints by repeating the liquid development on one electrophotographic sensitive paper or electrostatic recording paper while changing the toner color each time, reduction of the time required for each treatment is particularly important and is sought at all times. The present invention provides a developing method which meets these requirements in particular.

Another object of the present invention consists in providing a method for obtaining an image of continuous tone with a particularly high density through a treatment effected at a high rate of speed.

Still another object of the present invention is to provide a method suitable for obtaining an image through the combination developing method using liquid developing agents having equal magnitude and sign of electric charge.

One of the other important objects of the present invention resides in providing a developing method which does not involve the shortcoming of blurring (streaking) of the image.

DESCRIPTION OF THE INVENTION The present invention relates to a developing method for electrophotography comprising the steps of developing an electrostatic latent image formed on a photoconductive insulating layer with a liquid developing agent containing therein fine particles with positive electric charge and subsequently cleaning the insulative layer carrying the image thereon. The method is further characterized in that the liquid developing agent is composed of a carrier liquid, fine particles of positive electric charge contained in the dispersed state in the said carrier liquid, and a resin contained in the dissolved state in the carrier liquid, and (I) the carrier liquid has an electric resistance exceeding 10 9 cm. and is insoluble in ethyl cellulose, (II) the fine particles which are contained in the dispersed state are composed of pigment particles, insoluble in the carrier liquid, and ethyl cellulose particles, and (III) at least a part of the resin which is contained in the dissolved state in the carrier liquid is insoluble in the cleaning liquid.

This method is particularly desirable when (I) the carrier liquid has a composition such that more than 80% by volume thereof consists of a saturated hydrocarbon, which neither dissolves nor swells the photoconductive insulative layer, which has an electric resistance exceeding 10 cm., a dielectric constant not exceeding 3.5, and a kauri-butanol value exceeding 30, and which is insoluble in ethyl cellulose, (II) the fine particles, present in the dispersed state, are insoluble in the carrier liquid, are composed of pigment particles less than 1 in diameter and ethyl cellulose particles less than 0.5;]. in diameter, the pigment being present in an amount of from 0.05 to 0.0001 part by weight based on one part by weight of the carrier liquid and the ethyl cellulose present in an amount of from 0.01 to 0.4 part by weight based on one part by weight of the pigment, and (III) the resin contained in the dissolved state in the liquid is insoluble in a solvent having a kauri-butanol value less than 30 and the resin is present in an amount of from 0.00002 to 0.03 part by weight based on the carrier liquid.

A's the cleaning liquid, there is used isoparaffin or a chlorofiuorinated hydrocarbon having a kauri-butanol value not exceeding 30.

The aforementioned liquid developing toner to be used in the present invention is characterized in that it has a positive electric charge and is extremely stable with this electric charge and therefore is a stable dispersion over a long period.

Accordingly, this liquid developing agent produces attraction development (positive development) through coulombic forces when applied to the development of an electrostatic latent image having a negative charge. It produces repulsion development (reverse development) when applied to the development of an electrostatic latent image having a positive charge.

When particles of a substance insoluble in the carrier liquid are dispersed in the carrier liquid, these particles assume an electric potential with reference to the carrier liquid.

At this time, the magnitude and sign of the electric charge are determined by the condition of particle surface,

the particles adsorbed on the particle surface, and other factors.

Generally, when a hydrocarbon type of solvent is employed as the carrier liquid and particles are dispersed therein, the sign of the electric charge is proper to the substance being used.

For example, phthalocyanin blue, brilliant carmine 6B, Hansa Yellow, rosin gelatin, albumin, casein, acetocellulose, polyvinyl acetate, polyamide resins, nylon, polymethylmethacrylate, and the like, assume a positive electric charge. On the other hand, sulfur, selenium, phthalocyanin green, lead chromate, nitrocellulose, vinyl chloride type copolymers, polyvinylidene chloride, polyvinylidene fluoride, chlorinated polypropylene, phenyl modified alkyd resins, and the like assume a negative electric charge upon contact with a hydrocarbon type of solvent. Empirically, it is known that these electric charges substantially agree with the series of frictional electric charging.

For the preparation of a liquid developing agent containing positively charged particles to be used for developing an electrostatic latent image formed on a zinc oxide sensitive layer, it is naturally desirable to use a pigment possessing a positive electric charge or a resin possessing a positive electric charge as already mentioned. As a developing agent with a magenta color, it is suitable to use a preparation of brilliant carmine 6B dispersed in kerosene, for example. In the case of a preparation having only a pigment suspended and dispersed in a carrier liquid, it often happens that the electric charge possessed by the particles is unstable and the preparation consequently cannot be subjected to a prolonged storage. Where a multi-color image is obtained using developing agents of different colors, it is extremely difficult to make preparations of different colors having only pigments suspended therein and, at the same time, equalizing the electric charge of the suspended particles with different colors.

In such cases, addition of an electric-charge regulating agent or controller to the carrier liquids is usually attempted. First, the electric-charge controller is mixed intimately with a given pigment, and the resultant mixture is then added to and dispersed in the carrier liquid. Where the electric charge controller is soluble in the carrier liquid, the dispersion can be effected comparatively easily. However, a liquid developing agent which is prepared in this manner generally suffers from inferior storability so that the electric charge of the dispersed particles vary with age. The probable cause of this is that the electric-charge controller which is deposited on the particle surface initially gradually escapes into the carrier liquid.

The inventors have achieved the following advantages by permitting a resin soluble in the carrier liquid but insoluble in a solvent having a kauri-butanol value not exceeding 30 to be dissolved in the liquid developing agent and having finely divided ethyl cellulose particles dispersed therein in addition to the toner (pigment particles).

1) Improvement of the developing density and the durability of the resultant image against cleaning treatments including the use of squeeze rollers.

(2) A stable dispersion of the developing agent and a marked stabilization of the electrically charged condition of the toner.

It has been confirmed that these advantages are decidedly more conspicuous when the resin soluble in the carrier liquid and the ethyl cellulose particles are used in combination than when the two are added independently to the liquid developing agent.

At the present stage, the part which is played by the resin soluble in the carrier liquid and the ethyl cellulose particles is not yet understood. While not desiring to be bound it is believed that the following explanation accounts for the effects observed.

It appears that the ethyl cellulose particles, when allowed to come into contact with the toner within the liquid developing agent, function strongly to confer a stable, positive electric charge on the toner. When a microscopic observation is made on a preparation obtained by adding ethyl cellulose particles to the liquid developing agent which already contains a positively charged toner, no ethyl cellulose particle is found to exist in its isolated state. This phenomenon seems to have some bearing upon the assumption of a positive electric charge by the ethyl cellulose particles within the carrier liquid. A considerable magnitude of electric attraction due to polarization is considered to occur between the ethyl cellulose particles and the toner particles. Consequently, the ethyl cellulose particles seem to adhere to particles possessed of a negative electric charge and to those particles with a positive electric charge as well within the carrier liquid. As these particles assume such a state, they will come to possess a stabilized, strongly positive electric charge by virtue of the ethyl cellulose and, as a consequence, quickly adhere to the surface containing the latent image. Accordingly, an improvement of developing density and some increase in the mechanical strength of the image results.

These effects are enhanced even more by adding a resin soluble in the carrier liquid.

In the sphere surrounding the toner adhering to the surface of the latent image, the resin exists in the dissolved state accompanied by ethyl cellulose particles. This resin seems to be insolubilized when washed with a liquid possessed of a kauri-butanol value not exceeding 30, with the result that it will serve to immobilize the image further. Although the image is destroyed when it is rubbed while in the wet state immediately after the step of cleaning, it sufliciently withstands treatment with squeeze rollers.

Now, a description is made of specific embodiments of the present invention. The carrier liquid to be used for this invention must not be of the type which will dissolve or swell the photoconductive sensitive layer. The reason is that when the sensitive layer yields to such action, the electrostatic latent image is destroyed easily.

In the present invention, a resin which is insoluble in a solvent (cleaning liquid) having a kauri-butanol value not exceeding 30 is dissolved in the carrier liquid and is used in that state. In this connection, the carrier liquid should not be of the type having an extremely low dissolving power. It is naturally a prerequisite that the kauributanol value should not exceed 30. Further, the carrier liquid must possess a high electric resistance. Generally, it is said that the liquid developing agent has a resistance of a value exceeding 10 9 cm. In the case of the carrier liquid alone (containing neither the toner nor the resin), the value is required to exceed 10 cm. In the case where both resin and toner are contained in the carrier liquid, it is desirable that the liquid manifests a resistance exceeding 52 cm. In the case of the present invention, the carrier liquid is put to use with ethyl cellulose particles dispersed therein. As a result of research, it has been found that in an aromatic hydrocarbon, dispersion of ethyl cellulose becomes unstable. A possible cause for this is that ethyl cellulose is dissolved or swelled to some extent by the aromatic hydrocarbon. As the substance which completely satisfies the conditions dealt with above, there can be cited saturated hydrocarbons such as straight chain or aliphatic hydrocarbons. For example, cyclohexane, n-heptane, n-hexane, Decalin, kerosene, and mixed solvents thereof are suitable as the carrier liquid to be used for the purpose of this invention. Depending on the combination with the resin which is used in the dissolved state, a small quantity of an aromatic hydrocarbon or a chlorinated hydrocarbon can be added to such carrier liquid.

In this case, the quantity of aromatic hydrocarbon or chlorinated hydrocarbon should be within the limit beyond which the dispersion of ethyl cellulose is not obstructed, namely, it should not exceed 20% by weight based on the carrier liquid.

The resins which are soluble in the carrier liquid of the nature mentioned above but insoluble in the cleaning liquid having a kauri-butanol value not exceeding 30 include vegetable oil-modified alkyd resins, rosin-modified phenolformaldehyde resins, xyleneformaldehyde resins, polybutylmethacrylates, and styrene-butadiene copolymers. In this case, those resins having too large a molecular weight have a possibility of forming a strong coating during the cleaning treatment and therefore are diflicult to use. When the liquid developing agent is formulated, more desirable results can be obtained by first sufiiciently mixing and blending a given pigment with the resin to be used and subsequently adding the mixture to the carrier liquid.

With regard to the preparation of the liquid developing agent, numerous literature references and various patents are known. For use in the present invention, however, the toner particles (pigment) should not exceed 1p. in diameter. That is to say that among the particles that are deposited on the surface of the latent image, those of the pigment have a unit diameter not exceeding 1 1.. Pigment particles having diameters exceeding this limit have a tendency to come off the layer surface. Such pigment particles have electrophotographic properties which are unsatisfactory in many respects and it is extremely difficult to obtain desirable results with these pigment particles.

It is desirable that the fine particles of ethyl cellulose to be used for the present invention have a diameter smaller than that of the pigment particles. Where they have a diameter larger than the pigment particles, the developing density is lowered heavily and the effect to which the present invention is directed is diminished as well. Research has demonstrated that better results can be obtained by using particles of ethyl cellulose having a diameter less than one half, more preferably less than one quarter, of that of pigment particles. Fine particles of ethyl cellulose, such as mentioned above, can be obtained by the following method. As a preparatory step, ethyl cellulose is dissolved in a proper solvent (such as a chlorinated hydrocarbon or ester). Then, the resultant solution is added to and dispersed in a large amount of a medium incapable of dissolving ethyl cellulose with the application of ultrasonic waves. In this case, it is sufiicient to use the aforementioned carrier liquid as the medium incapable of dissolving the ethyl cellulose.

The dispersion of ethyl cellulose thus prepared can be added, in its unmodified state, to the carrier liquid of the present invention. For the purpose of such addition, it is not necessary to separate the ethyl cellulose from the dispersion particularly. In the dispersion of the ethyl cellulose which is obtained by the method just mentioned, particles of ethyl cellulose having diameters not exceeding 0.5 1. can be dispersed stably. This dispersion of ethyl cellulose can be added to the developing agent at any stage in the preparation of the liquid developing agent.

The toner comprising pigment particles is added in an amount corresponding to from 0.05 to 0.0001 part by weight based on one part by weight of the carrier liquid. When the toner quantity is too large, fogging tends to occur. When it is too small, the developing speed is lowered. A lowered developing speed is not desirable, because it results in a loss in the optical density of the image to be obtained or in the development of edging phenomenon.

The quantity of ethyl cellulose particles to be added to the pigment toner is also an important factor. When ethyl cellulose particles are used in too large a quantity, the image density is lowered, although the resistance of the image to the treatment of cleaning may be increased. Generally, the most desirable results are obtained when the ethyl cellulose is used in an amount corresponding to from 0.01 to 0.2 part per one part by weight of the pigment. In this case, the image density is improved as compared with the case in which no ethyl cellulose is added. Where ethyl cellulose is added in an amount of from 0.2 to 0.4 part, no additional improvement of image density is obtained, while the effect of increasing the resistance of the image to the treatment of cleaning remains unaffected. Generally, the quantity of the resin contained in the dissolved state in the carrier liquid is substantially the same as the toner contained therein (as pigment particles). Not all the resin contained in the dissolved state in the carrier liquid is required to be of the type which is insoluble in a solvent (cleaning liquid) having a kauri-butanol value not exceeding 30. However, the portion of the resin which accounts for at least 0.0002 to 0.03 part by weight based on one part by weight of the carrier liquid must be insoluble in the cleaning liquid. The lower limit of the quantity is determined by the resistance to the treatment of cleaning and to the squeezing motion, while the upper limit thereof is fixed according to the decrease in the electric resistance of the carrier liquid due to the addition of resin.

The cleaning liquid to be used for the purpose of this invention must be of the type having a considerably lower dissolving power than the carrier liquid. The kauri-butanol value of the cleaning liquid is preferably less than 30.

Use of a kauri-butanol value of 30 as the criterion in this respect relates to the technical level prevailing today. This value may be still higher so long as it is practical to prepare an electrophotographic sensitive layer possessing excellent resistance to solvents. As a natural consequence, the kind of resin and the amount of resin to be used would naturally be varied if the value was increased.

Suitable solvents for such purposes are saturated aliphatic hydrocarbons or chlorinated hydrocarbons with a side chain, having a boiling point over 40 C. and a solidifying point under C., or a mixture thereof with straight chain saturated hydrocarbons.

It might be thought that if insolubilization of resin occurs in the course of cleaning, there is a possibility that the fogging in the background area will become unremovable. Through experiments, such has been proven to be entirely unnecessary. In the area of fogging, pigment particles are not adhered strongly to the sensitive layer or insulating layer electrostatically and therefore are washed off by cleaning liquid even when insolubilization of resin occurs. Conversely, in the image area, pigment particles are electrostatically deposited on the layer surface strongly, so that upon contact with the cleaning liquid, the soluble resin component present between pigment particles is deposited in the sphere surrounding the pigment surface and functions to retain the particles on the surface, though relatively weakly. This type of fixing activity is not so strong. Often, it is the activity of such a degree that rubbing with a finger tip or a locally applied force will destroy the image easily. However, the activity is suflicient to withstand the force to which the image is exposed within the apparatus.

The cleaning liquid is permitted to contain the toner in an amount about one tenth of the amount in the developing liquid. This is because the latent image has already been developed substantially completely :and will no longer attract any toner existing in :a very small quantity.

In case where a color print or a multi-color image is obtained by the combination printing method, the toner image which is produced is dried and subsequently immersed again in the next developing liquid. In many cases, it happens that the carrier liquid in the second developing liquid is similar to that of the first developing liquid. As a natural consequence, it might be thought that the first image may be destroyed in the second developing liquid. However, it has been found experimentally that no such disadvantage occurs in actuality. A possible cause for this is that the first image is caused by the treatment with squeeze rollers and the process of drying to adhere to the sensitive layer with very strong force.

Where there is obtained an image of multiple colors with continuous tone, reduction of fogging in each of the developing stages will serve to improve the quality of the final print to a great extent.

Now, a more specific description is made of the present invention by referring to preferred embodiments thereof.

EXAMPLE 1 In a ball mill, the following ingredients were blended for 12 hours:

Photoconductive zinc oxide (Sazex made by Sakai Chemical Co.) g Vinyl chloride/vinyl acetate copolymer g 36 Ethyl-phthalyl ethyl glycolate g 4 Toluene cc 35 Butyl acetate cc 25 The white dispersion obtained on blending was adjusted in viscosity with butyl acetate, and spread, with a bar coater, to a dry thickness of 8p. on a paper treated so as to permit electroconductivity. The resultant sheet was allowed to stand at room temperature to dry sufficiently. Thereafter, it was preserved in a dark place at 40 C. for 20 hours so as to be adapted to darkness.

When the resultant sensitive layer was exposed to corona discharge at --7000 v., it was found that a surface potential of 450 v. was retained. (It was further found that after one minute of standing in a dark place, the sensitive layer suffered from a potential attenuation of 45 v.) The electrically charged sensitive layer was exposed to a light source having a color temperature of 2000 K. and a brightness of 5500 luxes through a positive image having continuous tone. Immediately after exposure to light, the sensitive layer was immersed in kerosene containing no toner, removed from the bath, held to allow excess kerosene to flow off, and then immersed in a bath of a liquid developing agent.

The liquid developing agent was prepared by the following method. As the first step, the following ingredients were blended sufficiently in a three-roll kneader.

Brilliant Carmine 6B g 25 Polymeric linseed oil g 25 Linseed oil-modified alkyd resin (with 70% of oil length) g 30 Kerosene ml 30 The linseed oil-modified alkyd resin used for this purpose was the type soluble in an isoparafiinic solvent.

One gram of the red paste obtained was dispersed in a liquid of the following composition:

Kerosene 300 Cyclohexane 700 The developing liquid thus prepared was a stable dispersion with a red color. The dispersed particles were so fine that no turbidity could be observed visually. In this liquid, the aforementioned sensitive layer was developed. After sixty seconds of standing in the bath, the sensitive layer was removed and then rinsed in a bath of refined kerosene so as to wash off excess developing agent wetting the surface thereof.

Consequently, there was obtained a clear, red positive image. The maximum reflective optical density was 1.72. In the vicinity of the image area, however, there appeared some fogging in a blurred state.

At that stage, the sensitive layer was wet with kerosene. Consequently, when this sensitive layer was squeezed The dispersion of ethyl cellulose was added in varying quantities to the developing agent of Example 2, and the resultant mixtures were used as developing liquids. Then the electrostatic latent image was developed in the same manner as in Example 1, and the sheets were washed with Isoper E. The sensitive layers still wet with Isoper E were squeezed through metallic rollers.

The results obtained are shown in Table 1. In the table, the developing agent of Example 2 is given as (I) and m the dispersion of ethyl cellulose as (II) respectively.

TABLE 1 Ghost Flowing Foggin image of image due to g I II Image due to due to develop- (percent) (percent) density squeezing squeezing ment 100 1.69 Slight sparing"... Present. 95 5 1.78 None None sparing. 90 1 85 ...--do o None. 80 1 76 do .do Do. 70 30 1 66 do do Do.

Example 1 1.72 Present Present"... Present.

sene. Accordingly, the image area became composed solely of pigment and was deprived of self-fixing characteristics. As a result, the action of squeezing gave rise to the aforementioned trouble.

The fogging in the blurred state that appeared in the vicinity of the image after the step of development (prior to cleaning) is believed to have been caused by the lack of stability of the electric charge assumed by the toner which was contained in the developing agent.

EXAMPLE 2 To one liter of the developing agent prepared in Ex ample 1, there was added 0.5 g. of a linseed oil-modified alkyd resin having an oil length of 52%. This linseed oilmodified alkyd resin with an oil length of 52% was soluble in kerosene but insoluble in isoparaffin and a chlorofluorinated hydrocarbon.

By following the same procedure as in Example 1, the electrostatic latent image was developed and subsequently washed with Isoper B (an isoparafiinie solvent having a kauri-butanol value of 29 made by Esso Standard Oil Co.). The sheet, still wet with Isoper E, was squeezed through metal rollers. Compared with Example 1, the blurring (expanding) of the image in the squeezing direction was reduced markedly but the ghost image due to partial transfer of image to rollers appeared to some extent. Prior to the step of squeezing, a fogging in the blurred state already appeared near the image area as in Example 1. The maximum reflective optical density in the image was 1.69.

EXAMPLE 3 A dispersion containing fine particles of ethyl cellulose was prepared in the manner described below.

As the first step, a solution having the following composition was prepared.

Ethyl cellulose g 1 Tetrachloroethane ml 150 Linseed oil-modified alkyd resin (having an oil length of 70%) g 2 Thirty (30) m1. of this solution was dispersed by means of ultrasonic waves in a liquid of the following compo sition.

M1. Cyclohexane 700 Kerosene 250 Linseed oil 50 In Table I, the data for Test No. 1 are those obtained in Example 2.

From the results shown above, it can be seen that the image obtained by adding a lineseed oil-modified alkyd resin (oil length 52%) and ethyl cellulose dispersion to the developing agent of Example I acquired a marked degree of resistance to the treatment of squeezing and an increased level of image density. The improvement in the resistance to the treatment of squeezing is believed to have resulted from the phenomenon that, because of washing in Isoper E, the linseed oil-modified alkyd resin (having an oil length of 52%) was insolubilized and consequently manifested self-fixing activity. The description fogging as used in Table 1 represents the fogging which appeared in the lblurred state in the vicinity of the image at the time of development. (Thus, the term refers to a characteristic somewhat different from the fogging as generally referred to.) This fogging was eliminated by the addition of the dispersion of ethyl cellulose.

EXAMPLE 4 In 1 liter of cyclohexane, 0.5 g. of blue offset printing ink was dissolved and dispersed. By following the procedure of Example 1, there was formed an electrostatic latent image, which was pre-treated in the bath of kerosene and then treated with the blue liquid developing agent. The blue image obtained consequently was washed with Isoper E and then passed through squeeze rollers. The image Was blurred slightly and a slight ghost image was produced. This trouble was eliminated completely when 0.2 g. of a butyl methacrylate/styrene copolymer was dissolved in the liquid developing agent and then ml. of the dispersion of ethyl cellulose of Example 3 was added thereto. The butyl methacrylate/styrene copolymer had a melting point of 75 C., and the molar ratio of butyl methacrylate to styrene was 50:50. This polymer was soluble in cyclohexane but insoluble in Isoper E and kerosene.

EXAMPLE 5 A dispersion containing fine particles of ethyl cellulose was prepared in the manner described below.

As the first step, a solution having the following composition was prepared:

Ethyl cellulose g 1 Varnish obtained by cooling a rosin-modified phenolformaldehyde resin with linseed oil g 5 Tetrachlorothane m1 150 the rosin-modified phenolformaldehyde resin component thereof was insolubilized when the varnish was added to Isoper E or kerosene.

Separately, 1 g. of black otfset printing ink was dissolved and dispersed in 1 liter of cyclohexane. When the development was carried out using this black liquid and following the same procedure as in Example 1, the image was quite unclear and the image density was as low as 0.83. When the sensitive layer was washed with Isoper E and then passed through squeeze rollers, the image flowed and there occurred a slight degree of ghost image.

When 200 ml. of the dispersion of ethyl cellulose containing the varnish shown in this example was added to 1 liter of this developing liquid and the development was carried out with this mixture in the same manner, there was obtained a clear, black image. The image density was 2.04.

When the sensitive layer was washed with Isoper E and then passed through squeeze rollers, the image was found to be free from blurring or any ghost image.

Similarly desirable effects can be obtained by using Diaflon S-2 (chlorofluorinated hydrocarbon solvent made by Daikin Industries Co.) in the place of Isoper E.

What is claimed is:

1. In a method for developing an electrostatic latent image on a photoconductive insulating layer comprising developing said latent image with a liquid developing agent containing fine particles having a positive electrical charge and thereafter cleaning the ins-ulative layer carrying the image with a cleaning liquid having a kauributanol value of not exceeding 30, the improvement which comprises said liquid developing agent consisting essentially of a carrier liquid, fine particles having a positive electrical charge dispersed in said carrier liquid and from 0.0001 to 0.05 part by weight, per parts by weight of carrier liquid, of a resin selected from the group consisting of vegetable oil-modified alkyd resins, rosin-modified phenolformaldehyde resins, xyleneformaldehyde resins, polybutylmethacrylates, and styrene-butadiene copoly- 12 mers dissolved in said carrier liquid; wherein said carrier liquid consists essentially of at least volume percent of a saturated hydrocarbon which does not dissolve or swell the photoconductive insulating layer, having an electrical resistance exceeding 10 ohm-cm; wherein said fine particles consist essentially of from 0.0001 to 0.05 part by weight, per part by weight of carrier liquid, of pigment particles having a diameter of less than 1 micron and from 0.01 to 0.4 part by weight, per parts by weight of said pigment particles, of ethyl cellulose particles having a diameter smaller than that of said pigment particles and less than 0.5 micron in diameter and insoluble in said carrier liquid; and wherein said resin is one which is completely soluble in said carrier liquid, wherein from 0.00002 to 0.03 part by weight, per part by weight of carrier liquid, of said resin is insoluble in said cleaning liquid, said carrier liquid and said cleaning liquid being different liquids.

References Cited UNITED STATES PATENTS 3,384,051 5/1968 Hunstiger 118637 3,419,411 12/1968 Wright 25262.1 3,356,497 12/1967 Moe et a1. 117-37 L X 3,337,340 8/1967 Matkan 25262.1 3,293,183 12/1966 'Matkan 25262.1 3,276,896 10/1966 Fisher 1-1737 L X FOREIGN PATENTS 743,492 9/ 1966 Canada 252-62.l 271,885 7/1964- Australia 25262.1 261,010 8/1961 Australia 252-62.l

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R. 

